<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .s{
            width: 500px;
            height: 500px;
            background-color: rgb(255, 0, 0);
        }
        .g{
            width: 200px;
            height: 200px;
            /* 半透明 */
            background-color: rgba(0, 200, 0, 0.5);
        }
    </style>
</head>
<body>
    <div class="s">
        <div class="g">

        </div>
    </div>
</body>
</html>